Regulation of Lactogenic Hormone Binding in Rat Liver by Steroid Hormones

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Abstract
The effects of testosterone propionate, medroxy-progesterone acetate and cortisol on the binding of ovine [125I]iodoprolactin to 100,000 .times. g particulate fractions from liver of normal and estrogen-treated female rats were measured. In untreated animals 6.7 .+-. 1.1% (SD) of the radioactivity added to 0.5 mg of membrane protein was specifically bound to the hormone receptor. Specific binding was significantly (P < .05) decreased after 7 daily doses of testosterone (1.0 mg) to 2.8 .+-. 1.4%, medroxyprogesterone (0.25 mg) to 2.7 .+-. 0.2% and cortisol (5.0 mg) to 3.1 .+-. 1.3%. The serum prolactin concentration, 4.2 .+-. 3.4 ng/ml in normal animals, was not affected by the hormone treatment. Ethinyl estradiol, 10 .mu.g/day for 7 days, increasing the binding of [125I]iodo-prolactin to 16.6 .+-. 6.0% and increased serum prolactin to 50.6 .+-. 11.5 ng/ml. Simultaneous administration of testosterone, medroxyprogesterone or cortisol with estradiol did not diminish the estradiol-induced increase in serum prolactin, but completely prevented the increase in prolactin binding. Testosterone or cortisol given to animals pretreated with estradiol suppressed prolactin binding from 16.4 .+-. 4.2% to < 2.5% after 48-72 h. Parallel results were obtained with 125I labeled-human growth hormone whereas 125I labeled-insulin binding was not affected by these treatments. Scatchard analysis showed that the decrease in lactogenic hormone binding was due to a reduced concentration of receptors with no significant change in affinity. Since serum levels of prolactin were not changed; treatment with testosterone, medroxyprogesterone and cortisol decreased lactogenic hormone binding by a direct action on the liver.

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